Especially popular embodiments of such heaters are those with cylindrical geometry and so-called electric flat heating elements which have an essentially flat geometry, which can be converted into almost any shape by reshaping in some embodiments. Heaters with cylindrical geometry have become known, for example, from DE 201 09 413 U1, EP 1 395 085 B1 or DE 103 33 206 B4.
Flat heating elements have become known, for example, from DE 72 25 341, DE 71 10, 931, DE 201 08 963 U1 and DE 201 18 050 U1.
The prior-art electric heaters usually consist of two contact areas arranged spaced apart from one another, between which is embedded a usually tubular heating element which comprises the actual heat conductor. From the state of the art are known essentially two approaches for achieving an as good as possible decrease in temperature during the embedding, namely the integral casting of the tubular heating element and insertion of the tubular heating element in grooves of a metal surface or insulating material surface. All these approaches have considerable drawbacks, as is explained below.
In the integral casting of the tubular heating element the problem of spontaneous bubble formation arises. This leads to defective decrease in temperature and failure of the heating element.
Insofar as it has been suggested to embed a heat conductor in a body being used as an intermediate layer or a contact area, especially a plate or a tube with recesses or grooves provided therein, the high labor effort that is accompanied by the production of grooves for embedding is unfavorable. Furthermore, it is difficult to guarantee in such an embedding that the thermal contact between the heating element and the insulation material plate is homogeneous, i.e., is equally good everywhere. If, however, this is not achieved, local defective decrease in temperature arises, which ultimately brings with it the failure of the heater.
Insofar as it has been suggested to press the heating element which is designed as a tubular heating element into contact areas made of metal lying on one another, a carrier structure or an enclosing sheet metal, it is problematic that the energy consumption which is needed to achieve a complete enclosing of the heating element is so high that the heating element is severely impaired in terms of its function. Deformations and compressions do arise, which lead to a local change in the ohmic resistance. Furthermore, an uncontrolled shifting of the coils of the heating element may occur, so that the desired temperature profile on the surface of the electric heating element is then no longer achieved. Finally, it has been shown that pressures to be applied are so high that the metal plates begin to flow, which leads to an undefined elongation of the electric heating element.
In addition, this is also not possible for any geometries of the heating element. Especially in tight coils with small pitches, a complete enclosure of the coils does not occur, i.e., a local, defective decrease in temperature, which is accompanied by the risks of failure already mentioned above, does not occur.